Structural adhesives have found broad use in the modern-day manufacturing environment. In the automobile industry, for example, the adhesive bonding of metals to plastics has found wide application where the use of rivets or other mechanical fasteners is impractical due to considerations of aesthetics, cost or corrosion (due to the use of dissimilar metals). Such adhesive bonding applications have found broader acceptance due to recent advancements in formulations of adhesives, which allow effective bonding to metals (see U.S. Pat. Nos. 4,293,665 and 4,452,944, both assigned to Lord Corp.), bonding to solvent-resistant plastics (see U.S. Pat. No. 4,714,730, assigned to ITW, Inc.) and for low temperature applications (see U.S. Pat. No. 4,942,201, assigned to ITW, Inc.). The teachings of these four references are hereby incorporated by reference.
The effectiveness and strength of these adhesives allows for the substitution of adhesives for mechanical connectors such as bolts and rivets. In metal-bonding applications, such mechanical connectors can have disadvantages, such as appearance, cost or corrosion due to the dielectric effects of dissimilar metals. Many manufacturing situations employ adhesives to effect the bonding of two substrates, such as plastic to metal and metal to metal for reasons of economy and aesthetics.
Acrylate and methacrylate adhesives are generally known in the industry. U.S. Pat. No. 4,536,546 assigned to ITW of Glenview, Ill., the contents of which are incorporated herein by reference, describes a methacrylate based adhesive which employs a methacrylate ester monomer with a aldehyde-amine catalyst, a chlorinated polyethylene polymer and a graft copolymer. U.S. Pat. Nos. 4,942,201 and 4,714,730, also assigned to ITW, the contents of which are incorporated herein by reference, describe adhesives made with methacrylate and acrylate based adhesives together with various other additives and ingredients for use in the construction of large objects.
Metal bonding adhesives are also described in the art. U.S. Pat. No. 4,452,944, assigned to Lord Corp., describes a formulation composed of acrylates and methacrylates monomers, polymerizable non-acrylic monomers (styrenes), isocyanate functional prepolymers reacted with polyols, phosphorous compounds, oxidation/reduction systems capable of effecting a room temperature cure, tertiary amines as accelerators, metal molybdates and polybasic lead salts of phosphoric acid. The use of urethane prepolymers and the absence of carboxylic acids in this reference is intended to improve resistance to heat degradation of the adhesive.
In view of the above, it is apparent that progress in the development of metal bonding adhesives is an ongoing process. However, there is still a continuing need for improved adhesive systems. Therefore, it is an object of the present invention to provide an improved two-part adhesive system.